1. Field of the Invention
The present invention relates to a filter cartridge including microporous filtration membranes. Particularly, it relates to a structure of a filter cartridge used for accurately filtering out fine particles and microorganisms of the order of 10 .mu.m or less, especially 1 .mu.m or less, such as filtration of refined or purified water for producing wash water used in the process of filtrating or producing medicines in the industrial pharmaceutical field, filtration of refined or purified water for producing hyperpure water used for fine processing in the field of electronics industry such as semiconductor manufacturing, or used in the process of producing alcohol drinks in the field of food industry, and other types of accurate filtration.
2. Background of the Invention
Heretofore, there has been widely used a pleat-type filter cartridge having a microporous filtration membrane. This filter cartridge is formed in such a manner that the microporous filtration membrane is creased so as to be shaped like pleats. A cylindrical filtration membrane structure having a height in the direction of the pleats is disposed in a parallel arrangement. The inside and outside of the cylindrical filtration membrane structure are protected by plastic members. Other plastic members are stuck to the ends of the cylindrical filtration membrane structure to be sealed to the membrane structure. The pleat-type filter cartridge has an advantage in that the area of the filtration membrane within a unit volume (effective filtration area) can be increased to thereby attain an enormous filtration flow rate per unit time. Therefore, the pleat-type filter cartridge is useful for large-scale filtration in the field of the aforementioned industries or in laboratories.
The filter must be installed in a housing for dedicated use. Recently, in order to save the trouble of installing the filter or for the purpose of small-scale filtration, a disposable filter cartridge (or so-called capsule-type cartridge) having an effective filtration area of 0.05 to 0.4 m.sup.2 is formed by uniting the filtration membrane structure and the housing to one body. This disposable filter cartridge has been used as one filtration means for a filtration line in which the filter must be frequently replaced in a short time after filtering various kinds of liquid. On the other hand, a so-called minifilter cartridge having a small effective filtration area and installed in a metal housing has been used for the purpose of small-scale filtration. The minifilter cartridge has a filtration membrane structure including a seal portion to be installed in the housing, but the filtration portion of the minifilter cartridge is substantially the same as that of the disposable filter cartridge. Both the disposable filter cartridge and the minifilter cartridge are generally classified into two types, viz., a pleat-type filter cartridge and a stratified plate-type filter cartridge formed by piling up plate-type filtration units, as disclosed in Japanese Patent Unexamined Publication No. 129016/1981. The structure and manufacturing method of both the disposable filter cartridge and the minifilter cartridge of the pleat type are substantially the same as those of the aforementioned pleat-type filter cartridge installed in the housing for dedicated use. In the case of the pleat-type disposable filter cartridge, the process of inserting the filtration membrane structure into the plastic housing is required after preparation of the filtration membrane structure. The structure and manufacturing method of the stratified plate-type filter cartridge in both the disposable filter cartridge and the minifilter cartridge are substantially as follows, as described in the Japanese Patent Unexamined Publication No. 129016/1981.
Two filtration membranes prepared by punching out the outer and center portions so as to be respectively shaped like a circle or a polygon are stuck to a platelike support having parallel planes. In the case of circle, each of the membranes is shaped like a doughnut. According to circumstances, the support is formed by sticking two plastic members together at the outer circumferential edge portions and inner circumferential edge portions or at other predetermined positions if necessary. Particularly, the outer circumferential edge portions and inner circumferential edge portions must be liquid-tightly sealed by sticking. Generally, the plate-like support is so designed that the flow path of filtered liquid is established by an inside space formed by sticking the two filtration membranes to both the sides of the support. In other words, the plate-like support forms a diaphysis structure having a center opening for out flow of filtrate at the inner circumferential edge portion of the support to thereby make it possible that the two filtration membranes can be held from the inside and that filtrated liquid can suitably flow out to the center portion of the plate-type filtration unit.
Further, a plurality of plate-type filtration units are stuck to each other by applying an adhesive to the center portions of the plate-type filtration units to prepare a filtration membrane structure stratified in the direction of thickness of the units. In order to protect the filtration membranes, an upper protective plate is mounted onto the upper portion of the stratified structure with a predetermined distance between the upper protective plate and the filtration membrane of the uppermost plate-like filtration unit. Similarly, a lower protective plate is mounted onto the lower portion of the stratified structure for the same purpose as described above. In the case of the minifilter cartridge, the lower protective plate serves also for the purpose of securing the setting of the housing. The thus prepared stratified filtration membrane structure is stuck to the housing to produce a disposable filter cartridge.
In the case of production of a minifilter cartridge, a seal portion having a slot for insertion of a sealing material, such as an O-ring or the like, to be set in the housing is provided in the stratified filtration membrane structure. The properties which are desired of the disposable filter cartridge and the minifilter cartridge are, for example, increasing the filtration area per unit volume, that is, providing a compact module of filtration, reducing filtration pressure, improving filtration accuracy, simplifying interchange of the filter cartridge into the filtration line, preventing the microporous membrane from being injured due to abrasion and impact in handling, reducing filtration cost, reducing the residual quantity of liquid, and the like.
However, the conventional plate-type filtration unit, for example, as disclosed in the Japanese Patent Unexamined Publication No. 129016/1981, has the structure in which the plate-like support has a large number of coaxial ribs, channels, and the like, and in which the filtration membrane is stuck to the ribs. If the filtration membrane is elastic, it expands to the primary side of filtration so as to be stretched when pressure from the secondary side acts on it for the purpose of backwashing or the like. In an extreme case, the filtration membrane is peeled from the support. As a measure to counter this, there may be provided means for increasing the area where the filtration membrane is stuck to the support. However, the portion where the filtration membrane is stuck to the ribs cannot function as a filtration membrane. There arises a problem in that the effective filtration area becomes relatively small compared to the whole area of the filtration membrane because the portion not functioning as a filtration membrane has a large area. Further, the structure in which the filtration membranes are stuck at a large number of places to both the sides of the plate-like support often causes distortion of the filtration membranes. Hence it becomes difficult to secure tightness at each of the outer circumferential edge portion and the inner circumferential edge portion between the respective filtration membrane and the plate-like support. Further, in the case where the structure is so compact as that of the present invention, the structure is difficult to produce and various processes are required. Hence, not only a problem exists in cost but also a problem arises in reliability of tightness and the like. Further, if the structure of the plate-like support, especially the structure of the liquid flow path composed of a large number of ribs, channels and the like, is so complex as disclosed in the Japanese Patent Unexamined Publication No. 219016/1981, the filtration resistance becomes large. Hence, not only a problem exists in that the liquid does not flow well, but also a problem exists in the escape of air and of residual liquid. In addition, the manufacturing cost and assembling cost of the plate-type support are both great.
Further, in the case where the plate-type filtration units are stuck to each other at their center portions so as to be piled up, adjacent filtration membranes often come into contact with each other if the filtration membranes are inclined. There exists the defect that the volume of the filter cartridge is increased as the distance between adjacent filtration units is increased to avoid such contact.